The present invention relates to a duct connector for a working fluid used in chemical plants or in such industrial fields as semiconductor manufacturing, liquid crystal manufacturing, and foods, and a valve having such duct connector; more particularly, it relates to a duct connector for a working fluid that has excellent rupture strength and excellent corrosion resistance, and a valve having the duct connector.
Conventionally, a resin cylinder housing used for a pneumatically driven valve has a construction where provided integrally on the peripheral side surface of a cylinder housing 44 is a duct connector 46 having a threaded fitting portion 45 communicating with the interior of the cylinder housing 44, as shown in FIG. 6. As is clear from the diagram, the female threading of the threaded fitting portion 45 has been provided up to the interior of the cylinder housing 44. The cylinder housing 44 has been made by injection molding, and to keep costs down, it has been necessary to mold the cylinder housing 44 without machining. Molding without machining requires the resin to have dimensional stability; in addition, use in a valve requires resistance to heat and chemicals. Resins that meet these requirements include glass-containing polyphenylene sulfide (PPS-G), polyvinylidene fluoride (PVDF-G), and polypropylene (PP-G); in particular, PPS-G has been used suitably for its dimensional stability. However, while offering excellent rigidity, PPS-G is also brittle. Thus generally, when connecting a nozzle for a working fluid to the threaded fitting portion 45 of the duct connector 46, in cases where the threaded fitting portion 45 is tapered, when the joint for working fluid is tightened, due to the taper, force is applied in the direction in which the duct connector 46 expands, and in cases where the threaded fitting portion 45 is straight, because sealing tape is wound around the joint for working fluid, force is applied in the duct connector 46 expanding direction. Therefore, to prevent breakage of the opening duct connector 46 portion of the cylinder housing 44, tightening is performed with a tightening torque of 0.4-0.5 Nm. Some workers, however, do not use torque wrenches, and apply a force greater than the prescribed torque, and PPS-G, which is both rigid and brittle, is unable to withstand a tightening torque greater than that prescribed, and the duct connector 46 portion breaks, so that even the cylinder housing 44 integral with the duct connector 46 breaks. In addition, if the cylinder housing 44 has been injection-molded, because of the structure of a channel communicating with the interior of the cylinder housing 44, a weld line will arise near the threaded fitting portion 45 of the duct connector 46. Because the weld line portion is weaker than other parts, when the nozzle for working fluid is connected to the duct connector 46, tightening with a greater-than-prescribed torque can easily cause breakage in the weld line portion; even after the nozzle for working fluid has been connected, an external load applied on the connection part of the nozzle for working fluid may cause breakage extending from the duct connector 46 to the cylinder housing 44.
To solve this problem, there have been inventions having a structure as shown in FIG. 7 (see, for example, patent reference 1). In this structure, an annular groove 49 is provided around a threaded fitting portion 48 formed in a plastic main housing 47, and a cylindrical metal insert 50 is inserted into the annular groove 49. The effect of the structure has been to strengthen the plastic threading 51 provided in the connection port 48 by reinforcing the metal insert 50.
Patent reference 1: Laid-open Japanese Patent Application H5-203078 (1993) (pages 5-6, FIG. 6)
However, with the threaded fitting portion 48 of the prior art, because the inserted metal insert 50 is exposed on the surface of the main housing 47, the cylindrical metal insert 50 can corrode, weakening the reinforcing effect, and in a corrosive atmosphere the threaded fitting portion 48 will break, entailing the problem that the threaded fitting portion cannot be used in such industrial fields as semiconductor manufacturing, in which chemicals that corrode metals are used. Further, if a rigid but brittle resin such as PPS-G is used for the main housing, because the plastic threading 51 and the main housing 47 are integrally formed, forceful tightening of the joint for working fluid may cause cracking in the plastic threading 51, with such cracks affecting the main housing 47, entailing the problem that the threaded fitting portion 48 may break.
In view of such problems with the prior art, it is an object of the present invention to provide a duct connector of a joint for working fluid that has excellent rupture strength and excellent corrosion resistance, and a valve comprising such a duct connector.